A wind turbine known in the art comprises a tapered wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor with a number of wind turbine blades is connected to the nacelle through a low speed shaft, which extends out of the nacelle front as illustrated on FIG. 1.
Modern wind turbines control the load on the rotor by pitching the blades in and out of the incoming wind. The blades are pitched to optimize the power output or to protect the wind turbine from damaging overloads.
To perform the pitch, each blade is provided with a pitching arrangement comprising a pitch bearing between the hub and the blade, and some sort of pitch mechanism, e.g., in the form of a hydraulic actuator, to provide the force for pitching the blade and maintaining it in a given position.
This hydraulic actuator is typically powered by a hydraulic pump which is driven by an electrical motor powered by the utility grid to which the wind turbine produces electrical power.
However, in case of grid failure, it can still be necessary to pitch the blades, for example, to shut down the wind turbine by feathering the blades, to control a slow rotation of the rotor during idling, or as a countermeasure in case one or more of the blades oscillates edgewise.
By way of example, from U.S. Pat. No. 4,348,155 it is therefore known to provide the hydraulic system with pressure accumulators so that the blades can be feathered in case of grid failure or a similar malfunction, but the power delivered from accumulators is limited and very short-lived.
From EP 1 788 237 A2 it is therefore known to power the hydraulic pump by the utility grid during normal operation and, for example, by means of a battery during grid failure but such a battery is both heavy and expensive.
One aspect of the invention is therefore to provide for an advantageous technique for powering the hydraulic pitch actuator of a wind turbine.